This invention relates to a vacuum belt conveyor of a web forming machine for transferring a threading tail, said vacuum belt conveyor comprising at least two turning rolls, an air-permeable belt loop arranged around the turning rolls, and vacuum means for providing a vacuum effect on both the belt loop section conveying the threading tail and in connection with the first one of said turning rolls.
Finnish patent application No. 20045069A discloses a vacuum belt conveyor according to the preamble, which is characterized by having a vacuum effect in connection with the first turning roll. Thus a negative pressure can be used to detach the threading tail from the surface of a dryer cylinder, for example. In the proposed vacuum belt conveyor negative pressure is created at the first turning roll by means of air blasting equipment arranged within the frame construction. Furthermore, a grooved roll is used as the first turning roll, via the grooves of which the vacuum effect is distributed over the entire turning length and essentially to the half of the circumference of the turning roll.
By using a grooved roll it is possible to achieve a relatively uniform distribution of negative pressure over the entire belt loop area contacting the turning roll. In practice, however, the belt loop is decidedly wider than the threading tail. This being the case, negative pressure escapes through the belt loop in the areas without the threading tail. Due to the incomplete coverage, negative pressure tends to balance over the entire turning length, whereby the maximum vacuum effect at the threading tail remains unachieved. In addition, the vacuum effect in the radial direction of the turning roll is essentially the same all over, although the critical positions vary between different applications. In practice, the grooved roll manufacture is also difficult. Furthermore, low and narrow grooves unnecessarily throttle the flow reducing thus the vacuum effect subjected to the threading tail. Regardless of the relatively wide vacuum belt conveyor, the threading tail can pass by at least partially in which case tail threading becomes unsuccessful. At the same time the threading tail may wind up around a dryer cylinder, for example, with disastrous consequences.